import pandas as pd
df=pd.DataFrame({
    'year':[2019,2020,None,2020,2022],
    'month':[3,None,6,9,12],
    'day':[4,2,21,2,15]
})

print(df.fillna({'year':2021,'month':9},inplace=True))
print(df)

df.drop_duplicates(keep=False,inplace=True)
print(df)

df['组合时间'] = pd.to_datetime(df)
print(df)